Diesel engines and other compression ignition engines are used to power light and heavy duty vehicles, locomotives, off-highway equipment, marine vessels and many industrial applications. Government regulations require the engines to meet certain standards for the exhaust emissions in each of these applications. Currently, the emission standards are for the nitrogen oxides NOx, hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM). Government agencies and industry standard setting groups are reducing the amount of allowed emissions in diesel engines in an effort to reduce pollutants in the environment. The environmental emissions regulations for these engines are becoming more stringent and difficult to meet, particularly for NOx and PM emissions. To meet this challenge, industry has developed many techniques to control the in-cylinder combustion process in addition to the application of after treatment devices to treat the engine-out exhaust gases and reduce the tail-pipe emissions. The emissions targets for the new production engines are even lower than the regulated emissions standards to account for the anticipated deterioration of the equipment during the life time of the engine after long periods of operation in the field. For example, proposed regulations for new heavy duty engines require additional NOx and diesel particulate emission reductions of over seventy percent from existing emission limits. These emission reductions represent a continuing challenge to engine design due to the NOx-diesel particulate emission and fuel economy tradeoffs associated with most emission reduction strategies. Emission reductions are also desired for the on and off-highway in-use fleets.
Traditionally, there have been two primary forms of reciprocating piston or rotary internal combustion engines. These forms are diesel and spark ignition engines. While these engine types have similar architecture and mechanical workings, each has distinct operating properties that are vastly different from each other. The diesel engine controls the start of combustion (SOC) by the timing of fuel injection. A spark ignited engine controls the SOC by the spark timing. As a result, there are important differences in the advantages and disadvantages of diesel and spark-ignited engines. The major advantage that a pre-mixed charge spark-ignited natural gas, or gasoline, engine (such as passenger car gasoline engines and lean burn natural gas engines) has over a diesel engine is the ability to achieve low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines is higher thermal efficiency.
One reason for the higher efficiency of diesel engines is the ability to use higher compression ratios than spark ignited engines because the compression ratio in spark ignited engines has to be kept relatively low to avoid knock. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels that are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion, a large fraction of the fuel exists at a very fuel rich equivalence ratio, which is known to lead to particulate emissions. A second factor is that the combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause higher NOx emissions. As a result, there is an urgent need to control the combustion process, not only to reduce the engine-out emissions, but also to produce the exhaust gas composition and temperature that would enhance the operation of the after treatment devices and improve their effectiveness.
The control of the in-cylinder combustion process can be achieved by optimizing the engine design and operating parameters. The engine design parameters include, but are not limited to engine compression ratio, stroke to bore ratio, injection system design, combustion chamber design (e.g., bowl design, reentrance geometry, squish area), intake and exhaust ports design, number of intake and exhaust valves, valve timing, and turbocharger geometry. For any specific engine design, the operating variables can also to be optimized. These variables include, but are not limited to, injection pressure, injection timing, number of injection events, (pilot, main, split-main, post injections or their combinations), injection rate in each event, duration of each event, dwell between the injection events, EGR (exhaust gas recirculation) ratio, EGR cooling, swirl ratio and turbocharger operating parameters.
Many types of after treatment devices have been developed, or are still under development to reduce the engine-out emissions such as NOx and PM in diesel engines. The effectiveness of each of the after treatment devices depends primarily on exhaust gas properties such as temperature and composition including the ratio between the different species such as NOx, hydrocarbons and carbon (soot). Here, also, the properties of the exhaust gases depend primarily on the combustion process.
The precise control of the combustion process in diesel engines requires a feed back signal indicative of the combustion process. Currently, the most commonly considered signal is the cylinder gas pressure, measured by a quartz crystal pressure transducer, or other types of pressure transducers. The use of the cylinder pressure transducers is limited to laboratory settings and can not be used in the production engine because of its high cost and limited durability under actual operating conditions.